The term litmus test is commonly used to mean a totally definitive test. In chemistry litmus is used ...

The term litmus test is commonly used to mean a totally definitive test. In chemistry litmus is used as an acid-base indicator. Which of the following properly relates the chemical and common usages of the term litmus test?
 a. Common usage is correct because the litmus test definitively defines a substance as an acid or a base.
  b. Common usage is correct because litmus dye changes color at pH = 7
  c. Common usage is incorrect because litmus dye changes color over a range, most of which is above pH = 7
  d. Common usage is incorrect because litmus dye changes color over a range, most of which is below pH = 7